try looking through /var/log/Xorg.0.log file and grepping for errors. did you do any updates recently? What was the last thing done on both machines that you can think of? What runlevel is specified in your /etc/inittab?
[root@centos mnt]# cat /var/log/Xorg.0.log|grep -i error
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
The runlevel was 3
I used to do startx from the command prompto to load gui
The last one was ctrl+alt+backspace on one system
and switching to another terminal and stoppiing the gdm i think